A stepper aligner system geometry typically comprises in order a light source, aperture blades, a reticle, a lens and finally the wafer. Adjustable aperture blades or blinds mask light from passing through parts of the reticle and lens so that less than the maximum field can be exposed on the wafer.
"Best focus" is the focus setting used on a step and repeat projection aligner that will provide the steepest resist sidewall slope and thus the best resolution and linewidth control. The stepper focus setting corresponds to the adjustable distance between the wafer surface and the reticle/lens.
In conventional focus determination, one or more sets of resolution targets in each of 25 to 80 fields, each exposed at incrementally different focus and exposure combinations, are evaluated microscopically (at 200.times. to 500.times. magnification). The nominal dimension of the smallest resolved feature is recorded on a data sheet. The matrix of numbers forms nested parabolas (curves of constant dimension) whose low exposure apexes define "best focus". Underexposure of positive photoresist provides the several parabolas for consideration. The error of this method is estimated to be .+-.0.75 .mu.m. A typical time to complete this qualification and determine the best focus value is about 25 to 30 minutes.
A more accurate method of interpretation of such a focus exposure matrix is to actually measure critical dimension bars in each field. Best focus is found at the center of the smallest bars resolved. However, this method is even more time consuming than that described above.
The foregoing procedure is commonly undertaken at the beginning of a work shift, and thus production is held in abeyance pending determination of best focus. Also, any changes that occur during the shift that could change best focus, such as barometric pressure changes or certain equipment maintenance procedures, can require that a redetermination of best focus be made.
For lens characterization, the procedure is similar, except that a maximum size field is exposed at different focus settings, and resolution bars are read at five or more locations on each field. For each location, the date comprising the nominal size of the smallest resolved resolution bars versus focus form a parabola whose apex is at best focus for that location. The difference in best focus values for different areas is caused by field curvature and lens tilt. The difference in best focus values for horizontal and vertical lines is due to astigmatism. Both must be accommodated in the process focus budget.
Limitations of the prior art lens characterization method include (1) accuracy limited by local resist, underlying film, and exposure variations at the site of the resolution bars, (2) operator fatigue limited by the number of locations that can be tested, so that a complete lens map can only be estimated, and (3) time requirements of the method and the replication of the test to verify results.
A need remains for a method of determining the best focus at one or more locations in a more rapid manner.